An investigation into the deaths of the three horses was launched in March 2012, at the urging of People for the Ethical Treatment of Animals (PETA).

The Hollywood Reporter has now revealed further difficulties for the network, as former American Humane Association employee Barbara Casey has sued HBO bosses for wrongful termination, after she apparently attempted to expose the alleged cover-up of the sub-par treatment of animals.

Casey served as director of production in the AHA's film and television unit for over 13 years, but was let go in January 2012 during the investigation into the animal deaths.

She claims that during her investigation she saw several drugged, underweight and/or sick horses on set, many of which were misidentified by producers to make it harder for animal rights representatives to track.

Casey goes on to suggest that HBO executives pressured the American Humane Association to turn a blind eye to some of the animal safety violations on set.

HBO has responded to the lawsuit in a statement, saying: "We took every precaution to ensure that our horses were treated humanely and with the utmost care, exceeding every safeguard of all protocols and guidelines required of the production.

"Barbara Casey was not an employee of HBO, and any questions regarding her employment should be directed to the AHA."